Diffing Local and Cluster State

Motivation

The ability to view what changes will be made before applying them to a cluster can be useful.

Generating a Diff

Use the diff program in a user's path to display a diff of the changes that will be
made by Apply.

kubectl diff -k ./dir/

Setting the Diff Program

The KUBECTL_EXTERNAL_DIFF environment variable can be used to select your own diff command.
By default, the "diff" command available in your path will be run with "-u" (unified) and "-N"
(treat new files as empty) options.